This invention relates to a method for obtaining drain-cast hollow articles. More particularly, this invention relates to a method for drain-casting a slip to obtain substantially hollow molded articles. Such hollow articles drain-cast with a ceramic slip are especially useful, which are then fired to produce substantially hollow ceramic or porcelain ware such as sanitary ware, art ware, other ceramic ware vessels, and the like. Moreover, such hollow articles having complicated appearances can be efficiently produced according to the present invention.
Conventional methods for drain-casting slips comprise, for example, filling the mold cavity of a divisible gypsum mold which has been dried with a slip, depositing the slip onto the surface of the gypsum mold by the action of gypsum in absorbing water from the slip, draining undeposited slip remaining in the mold by gravity, increasing the strength of the resulting molded article by having the moisture of the deposited slip absorbed into the gypsum mold, and then removing the mold by hand operations to obtain the molded article. Such conventional methods, however, produce only two or less molded articles in 8 hours due to the limited water-absorption capacity of a dried gypsum mold, and the used gypsum mold which has absorbed water needs drying for 6 to 18 hours before it can be reused. Moreover, the production capacity per working period is very low and also the use-life of such gypsum molds is short (about 80 cycles) due to the deterioration which occurs in prolonged drying. Thus, the production cost of such molded articles is considerably high.
In order to avoid troublesome manual operations and prolonged molding and demolding time as mentioned above, the present inventor proposed, as one of joint inventors, an automatic method for obtaining drain-cast hollow articles without substantial manual operations, as disclosed in U.S. Pat. No. 4,528,152 Specification the disclosure of which is incorporated herein by reference. The Aoyama et al. U.S. Patent is summarized below with reference to FIG. 5 (Prior Art) attached hereto. An apparatus for drain-casting a slip comprises a mold (1) including at least two divisible mating mold portions (2a, 2b) to form a mold cavity (4), a slip tank, a pump, air compressors, suction pumps, valves and tubings to connect them accordingly, supporting means (8) of the mold portions, and a stand. The mold portion (2a, 2b) includes an air-tight vessel (2) and a filter member (3, 3') inside the vessel, the filter member (3, 3') contains a water-drainage means (5, 5'), and the drainage means is communicated with the outside of the vessel. One of the mold portions has a slip supply duct (6) and one other mold portion has an overflow duct (7), which ducts communicate with the outsides of the vessels. A method using such apparatus to form hollow ceramic ware comprises pressurizing a slip introduced into the mold cavity (4) and preferably depressurizing the water-drainage means (5, 5') to deposit the slip onto the filter members (3, 3'), draining an undeposited slip through the slip supply duct (6), removing one of the mold portions by applying compressed air to the water-drainage means (5, 5') of the mold portion to exude some water between the filter member and the resulting molded article, depressurizing the water-drainage means (5, 5') of the other mold portion to attract the molded article into the mold portion, hanging the molded article attracted into the mold portion, applying compressed air to the water-drainage means (5, 5') as described above, and thus demolding the molded article on the stand (9) safely.
Most defects observed in conventional manual operations have been eliminated by the U.S. Pat. No. 4,528,152 invention. Strictly speaking, however, there still remain some problems to be improved in automatically demolding hollow molded articles having large dimensions or having complicated shapes. When a heavy hollow article having large dimensions is hung from a mold portion, sometimes cracks or breakage is caused by its weight or deformation due to shrinkage takes place in the unsupported (demolded) portions of the molded article. In order to avoid such problems, longer deposition time or curing time is required. Moreover, it is sometimes difficult to demold a hollow article having complicated shapes such as concave configurations. Actually, it has been believed in the art that a large hollow article having complicated shapes such as stool sanitary ware can not be produced by an automatic demolding process without manual operations.